Wafer processing method

Abstract

Disclosed herein is a wafer processing method including a first modified layer forming step of applying a laser beam having a transmission wavelength to a wafer from the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer near the front side thereof, thereby forming a first modified layer inside the wafer along each division line. The wafer processing method further includes a second modified layer forming step of applying the laser beam to the wafer from the back side thereof along each division line in the condition where the focal point of the laser beam is set adjacent to the first modified layer thereabove toward the back side of the wafer, thereby forming a second modified layer for growing a crack from the first modified layer toward the front side of the wafer.

What is claimed is: 
     
       1. A wafer processing method for dividing a wafer into a plurality of individual device chips along a plurality of crossing division lines formed on a front side of said wafer, the front side of said wafer being partitioned by said division lines to define a plurality of separate regions where a plurality of devices are formed, said individual device chips corresponding to said respective devices, said wafer processing method comprising:
 a first modified layer forming step of applying a laser beam having a transmission wavelength to said wafer from a back side thereof along each division line in the condition where the focal point of said laser beam is set inside said wafer near the front side thereof, thereby forming a first modified layer inside said wafer along each division line; and 
 a second modified layer forming step of applying said laser beam to said wafer from the back side thereof along each division line in the condition where the focal point of said laser beam is set adjacent to said first modified layer thereabove toward the back side of said wafer after performing said first modified layer forming step, thereby forming a second modified layer for growing a crack from said first modified layer toward the front side of said wafer; 
 wherein the focal point of said laser beam in said second modified layer forming step is set at a position slightly displaced by a predetermined offset amount from the position directly above said first modified layer in a Y direction perpendicular to an X direction corresponding to the scanning direction of said laser beam, thereby defining the direction of extension of said crack growing from said first modified layer toward the front side of said wafer. 
 
     
     
       2. The wafer processing method according to  claim 1 , further comprising:
 a back grinding step of grinding the back side of said wafer in the condition where a protective member is attached to the front side of said wafer, after performing said second modified layer forming step, thereby reducing the thickness of said wafer to a predetermined thickness and simultaneously dividing said wafer into said individual device chips. 
 
     
     
       3. The wafer processing method according to  claim 1 , wherein said offset amount is set to 6 to 8 μm.